1. Field of the Invention
The present invention relates to a semiconductor device including a resistor circuit made of polycrystalline silicon.
2. Description of the Related Art
In a semiconductor integrated circuit a diffusion layer resistor, which is made from a single crystalline silicon semiconductor substrate to which impurities of an opposite conductivity to the semiconductor substrate conductivity are introduced, and a polycrystalline silicon resistor, which is made of polycrystalline silicon to which impurities are introduced, are used. Of these, the polycrystalline silicon resistor is widely used in the semiconductor integrated circuit because of its advantages in, for example, a small leakage current due to an insulating film surrounding the resistor and in realization of a high resistance owing to defects existing at grain boundaries.
FIGS. 2A and 2B show a schematic plan view and a schematic cross-sectional view of a conventional polycrystalline silicon resistor circuit.
The polycrystalline silicon resistor is manufactured by introducing p-type or n-type impurities into a polycrystalline silicon thin film deposited on an insulating film by low pressure chemical vapor deposition (LPCVD) or the like, and then forming the resultant into a resistor shape by a photolithography technique. Impurity introduction is carried out to determine a resistivity of the polycrystalline silicon resistor. P-type impurities such as boron or BF2, or n-type impurities such as phosphorus or arsenic are introduced at a concentration ranging from 1×1017/cm3 to 1×1020/cm3 depending on a desired resistivity. Further, each terminal on both ends of the resistor is formed by a contact hole and metal line to pick up a potential thereof. In order to obtain a satisfactory ohmic contact between the polycrystalline silicon and the metal line at the terminal, impurities at a high concentration of 1×1020/cm3 or more are introduced into a part of the polycrystalline silicon corresponding to the terminal of the resistor.
Accordingly to construct a resistor circuit including resistor groups 201 to 204 illustrated in FIG. 3, the resistor 3 using the polycrystalline silicon is formed on an insulating film 2 disposed on a semiconductor substrate 1, and is including a low concentration impurity region 4 and a high concentration impurity region 5 as illustrated in the schematic plan view of FIG. 2A and the schematic cross-sectional view of FIG. 2B. A potential of each of terminals A (101) to E (105) is picked up by metal line 7 through a contact hole 6 provided on the high concentration impurity region 5. Further, in a case where various potentials are picked up from the resistor circuit, the resistor groups 1 (201) to 4 (204) are selected from resistor groups having various structures obtained by connecting a unit resistor in series or in parallel. Then, in order to stabilize a resistance for each resistor group, a metal portion is formed on the resistor group and connected to a terminal at an end of the resistor group. There are two reasons for the structure.
The first reason is to obtain a stability of the polycrystalline silicon resistor. Since the polycrystalline silicon is a semiconductor, formation of a metal line or an electrode causes depletion or accumulation of the polycrystalline silicon owing to a relative relationship between a potential of the metal line or the electrode and a potential of the polycrystalline silicon resistor, which changes the resistance of the polycrystalline silicon resistor. Specifically, presence of a metal line or an electrode having a higher potential than that of the polycrystalline silicon resistor directly above the polycrystalline silicon, into which the p-type impurities are introduced, causes the depletion of the p-type polycrystalline silicon, increasing the resistance of the polycrystalline silicon resistor. In a case of a reverse potential relationship, the resistance thereof is reduced owing to the accumulation. In order to avoid the resistance variation described above, a metal line having a potential close to that of the polycrystalline silicon is intentionally formed on the polycrystalline silicon, whereby a constant resistance can be maintained, which is illustrated in the schematic plan view of FIG. 2A as an example. In FIG. 2A, an electrode on one side of the polycrystalline silicon resistor is extended up to a resistor to fix the potential.
This phenomenon depends not only on the metal line above the polycrystalline silicon but naturally also on the metal line below the polycrystalline silicon. In other words, a relative relationship between potentials of the polycrystalline silicon resistor and a semiconductor substrate located below the polycrystalline silicon resistor varies the resistance. In view of this, there is known a method of stabilizing the potential by intentionally forming a diffusion region (not shown) or the like below the polycrystalline silicon resistor similar to the above-mentioned metal line.
The second reason is to prevent diffusion of hydrogen, which affects the resistance of the polycrystalline silicon, into the polycrystalline silicon in a semiconductor manufacturing process. The polycrystalline silicon includes a grain having relatively high crystallinity and a grain boundary between the grains which has low crystallinity, that is, a high level density. The resistance of the polycrystalline silicon resistor is mostly determined by electrons or holes serving as carriers which are trapped by a large number of levels existing at the grain boundary. Accordingly, generation of hydrogen having a high diffusion coefficient occurs in a semiconductor manufacturing process, the generated hydrogen easily reaches the polycrystalline silicon and becomes trapped by the level, which varies the resistance.
Examples of the hydrogen generating process described above include a sintering step in a hydrogen atmosphere after metal electrode formation and a formation step for a plasma nitride film using an ammonia gas. When the metal line covers the polycrystalline silicon resistor, the resistance variation of the polycrystalline silicon due to the hydrogen diffusion can be suppressed. The method of stably providing the resistance of the polycrystalline silicon is disclosed in, for example, JP 2002-076281 A.
The conventional method of stabilizing the resistance of the polycrystalline silicon as described above has, however, the following problem. Specifically, the metal portion on the polycrystalline silicon is susceptible and receives effects caused by, for example, heat, stress, and charge due to plasma, other than hydrogen which affect the polycrystalline silicon in the semiconductor manufacturing process. Accordingly these effects operate on the polycrystalline silicon through the metal portion thereon, resulting in the resistance variation.